This invention relates to a cryoablation apparatus for treating biological tissues, and more particularly, to a cryoablation apparatus having an enhanced heat exchange distal end section.
Cryosurgical therapy involves application of extremely low temperature and complex cooling systems to suitably freeze the target biological tissues to be treated. Many of these systems use cryoprobes or catheters with a particular shape and size designed to contact a selected portion of the tissue without undesirably affecting any adjacent healthy tissue or organ. Extreme freezing is produced with some types of refrigerants that are introduced through the distal end of the cryoprobe. The distal surface of the cryoprobe is desirably in direct thermal contact with the target biological tissue to be treated.
In many situations, however, cryoablation of biological tissue requires a desired target temperature within the target tissue which is not in direct thermal contact with the cryoprobe. In such situations, the distance the target tissue is from the actual cryoprobe or cryocatheter is important. For example, deeper cancerous tumors seen by imaging (e.g., ultrasound, computed tomography, magnetic resonance) will generally be killed by two freeze cycles to a target temperature of −40° C. with an intervening passive thaw cycle. The faster that the −40° C. target temperature is achieved throughout the tumor, the greater the lethality or cytotoxicity of each freeze to the tumor. Assuming approximately one cryoprobe for each centimeter of tumor diameter, the usual freeze time is up to 10 min each, within interval passive thaw of 5 min., for a total of up to 25 min with current clinical cryotechnology. The visible ice margin of 0° C. thus generally needs to extend beyond 1 cm of tumor margins to achieve the target temperature −40° C. beyond all tumor margins. There is a great need for improving the speed of these procedures, the thermal conduction of target temperatures to deeper tissues further from the cryoprobe, as well as limiting the number of cryoprobes needed to cover a target tumor volume.
There are various known cryosurgical systems including, for example, liquid nitrogen and nitrous oxide type systems. Liquid nitrogen has a very desirable low temperature of approximately −200° C., but when it is introduced into the distal freezing zone of the cryoprobe which is in thermal contact with surrounding warm biological tissues, its temperature increases above the boiling temperature (−196° C.) and it evaporates and expands several hundred-fold in volume at atmospheric pressure and rapidly absorbs heat from the distal end of the cryoprobe. This enormous increase in volume results in a “vapor lock” effect when the internal space of the mini-needle of the cryoprobe gets “clogged” by the gaseous nitrogen. Additionally, in these systems the gaseous nitrogen is simply rejected directly to the atmosphere during use which produces a cloud of condensate upon exposure to the atmospheric moisture in the operating room and requires frequent refilling or replacement of the liquid nitrogen storage tank.
Nitrous oxide and argon systems typically achieve cooling by expansion of the pressurized gases through a Joule-Thomson expansion element such as a small orifice, throttle, or other type of flow constriction that are disposed at the end tip of the cryoprobe. For example, the typical nitrous oxide system pressurizes the gas to about 5 to 5.5 MPa to reach a temperature of no lower than about −85 to −65° C. at a pressure of about 0.1 MPa. For argon, the temperature of about −160° C. at the same pressure of 0.1 MPa is achieved with an initial pressure of about 21 MPa. The nitrous oxide cooling system is not able to achieve the temperature and cooling power provided by liquid nitrogen systems, but has some advantages because the inlet of high pressure gas at room temperature. When nitrous oxide or argon it reach the Joule-Thomson throttling component or other expansion device at the probe tip, cooling along the shaft and extension hoses are limited, which precludes the need for heavy thermal insulation of those system components. However, because of the insufficiently low operating temperature, combined with relatively high initial pressure, cryosurgical applications are strictly limited.
Additionally, the Joule-Thomson system typically uses a heat exchanger to cool the incoming high pressure gas using the outgoing expanded gas in order to achieve the necessary drop in temperature by expanding compressed gas. These heat exchanger systems are not compatible with the desired miniature size of probe tips that need to be less than 3 mm in diameter. Although an argon system is capable of achieving a desirable cryoablation temperature, argon systems do not provide sufficient cooling power and require very high gas pressures and volumes. These limitations are very undesirable for practical clinical applications.
Another cryoablation system uses a fluid at a near critical or supercritical state. Such cryoablation systems are described in U.S. Pat. Nos. 7,083,612 and 7,273,479. These systems have some advantages over previous systems. The benefits arise from the fluid having a gas-like viscosity. Having operating conditions near the critical point of nitrogen enables the system to avoid the undesirable vapor lock described above while still providing good heat capacity. Additionally, such cryosystems can use small channel probes.
However, challenges arise from use of a near-critical cryogen in a cryoablation system. In particular, there is still a significant density change in nitrogen once it is crossing its critical point (about 8 times)—resulting in the need for long pre-cooling times of the instrument. The heat capacity is high only close to the critical point and the system is very inefficient at higher temperatures requiring long pre-cooling times. Additionally, the system does not warm up (or thaw) the cryoprobe efficiently. Additionally, near-critical cryogen systems require a custom cryogenic pump which is more difficult to create and operate at cryogenic temperatures.
Still other types of cryosystems are described in the patent literature. U.S. Pat. Nos. 5,957,963; 6,161,543; 6,241,722; 6,767,346; 6,936,045 and International Patent Application No. PCT/US2008/084004, filed Nov. 19, 2008, describe malleable and flexible cryoprobes. Examples of patents describing cryosurgical systems for supplying liquid nitrogen, nitrous oxide, argon, krypton, and other cryogens or different combinations thereof combined with Joule-Thomson effect include U.S. Pat. Nos. 5,520,682; 5,787,715; 5,956,958; 6074572; 6,530,234; and 6,981,382.
Another type of cryoprobe is described in US Patent Publication 20080119840 to Vancelette. A cryoprobe tip has an increased surface area by having a corrugated, waved, or otherwise ridged configuration in its inner and outer surfaces. The cryoprobe, however, is shown having complex tubular cross-sections which may be difficult to manufacture. The complex cross-sections of the tube portion shown in Vancelette may complicate the return path of the refrigerant thus making heat exchange inside the probe less efficient.
Despite the above patent literature, an improved cryoablation apparatus having a small size and shape to achieve selective cooling of the target biological tissue is still desired. The more rapid cooling of target tissues to cytotoxic temperatures at distances of several millimeters from the point of tissue contact is crucial, but is not attained by cooling capacity or low probe surface temperatures. Cryogenic systems with high cooling capacities, such as liquid nitrogen, near critical or single phase liquid cooling systems require faster and more reliable cryoablation procedures.
An improved cryoablation apparatus having a tip that can be placed in direct contact with the target biological tissue to be thermally treated, and to form an ice ball on the target tissue for a controlled period of time, and that increases the effectiveness of the cryosurgical treatment is still desired.
An improved cryoablation apparatus having cryoablation tip which can operate with a single phase liquid refrigerant is still desired.